In conventional propellor-operated boat propulsion systems, the centrigual movement imparted to the water by the rotating propellor tends to reduce the overall efficiency of the propulsion system. To offset this, various propellor shrouds have been proposed for circumferentially confining the water displaced by the propellor, but many such shroud arrangements have increased appreciably the drag on the boat.
In recent years boats have been provided with various jet propulsion systems which have the disadvantage that the water used for propulsion must undergo an abrupt change in direction, with a consequent waste of power.